An organic electroluminescent device emits light when the positive holes injected from an anode and the electrons injected from a cathode are recombined in an organic luminescent layer kept between both the electrodes. As a typical structure of it, as shown in FIG. 2, first electrodes 2 formed on a substrate 1, thin film layers including at least a luminescent layer 5 composed of organic compounds, and second electrodes 6 are formed on it, and the light emitted by such recombination goes out through the transparent electrodes to outside. Such an organic electroluminescent device is thin and allows high-brightness light emission at a low driving voltage or multi-color light emission by selecting the organic compounds of the luminescent layer, being applied as a luminescent device or display, etc.
Manufacturing an organic electroluminescent device, it is necessary to form a pattern of the luminescent layer, etc., and various manufacturing methods have been studied. In the case where fine patterning is required, photolithography is used as a typical method. For forming the first electrodes of the organic electroluminescent device, photolithography can be applied, but for forming the luminescent layer and the second electrodes, it is often difficult to apply photolithography, since photolithography basically has problems owing to a wet process. Therefore, for forming the luminescent layer and the second electrodes, a dry process such as vacuum deposition, sputtering, or chemical vapor deposition (CVD) is applied. Among these processes, as a means for patterning a thin film, a mask deposition method using a deposition mask is often applied.
The pattern of the luminescent layer of an organic electroluminescent device used as a display has a very high preciseness. In a passive matrix system, the luminescent layer is formed on the first electrodes patterned as stripes, and the line width of the first electrodes is usually 100 μm or less, while the pitch of the first electrodes is about 100 μm. The second electrodes are also formed as stripes to cross the first electrodes at a pitch of hundreds of micrometers, and the slender electrodes must have a low electric resistance in the lengthwise direction while the electrodes adjacent to each other in the crosswise direction must be perfectly insulated from each other. Also in an active matrix system, the luminescent layer is patterned at an equivalent or higher precision.
Therefore, the deposition mask used for patterning the luminescent layer is also inevitably required to have a high precision. The methods for manufacturing a mask member include etching, mechanical grinding, sand blasting, sintering, laser processing, use of a photosensitive resin, etc. Etching and electrocasting are often used, since they are excellent in the precision of fine patterning.
Further, if the mask member is thick, shadowing due to the deposition angle occurs and makes the pattern blunt. So, if a higher precision is required, the thickness of the mask member must be thinner. The thickness of the mask member for the luminescent layer is usually as thin as 100 μm or less, and it is generally fixed and held on a frame like a window frame when it is used in a deposition step.
The mask member of the deposition mask used for forming the luminescent layer has a mask area 7 and an opening area 9 demarcated by the outer edges of the openings 10 arranged for patterning (FIG. 3). This mask has a problem that an in-plane stress difference occurs between the mask area and the opening area depending on mask manufacturing conditions, and that local bending occurs at the boundary portion {the dotted line portion of FIG. 3 (a)}. If such a deposition mask is used, the closeness between the substrate and the deposition mask is impaired at the bent boundary portion between the mask area and the opening area, to blur the luminescent layer pattern for example. Especially in the case where the pitch of pixel sets, each set consisting of one each luminescent pixel of respective colors, is 500 μm or less, the colors of adjacent luminescent pixels are likely to be mixed, not allowing fine light emission to be obtained. This problem is more likely to occur for exerting larger influence when the boundaries are straight and longer, because of the nature of bending at the boundary portion between the mask area and the opening area. That is, this problem is more outstanding when the screen is larger-sized with longer sides.
To address the problem, known are a technique in which a tensioned mask member is fixed for the purpose of inhibiting its warping and bending and a technique as shown in FIG. 4 in which reinforcing wires 11 are partially introduced for the purpose of maintaining a patterning precision (for example, see Patent Document 1). However, these techniques are not able to inhibit local bending. Further, as the deposition mask for forming the second electrodes in conformity with a predetermined pattern, disclosed is a means for lessening the applied tension by dividing the mask member (for example, see Patent Document 2), but this technique is considered to be insufficient for higher precise patterning of the luminescent layer. Meanwhile, the positions at which said reinforcing wires are introduced are the positions overlying on the insulation layer, to avoid the influence on light emission. For this reason, in the case where a deposition mask with reinforcing wires is used, if the luminescent layer pattern has, for example, stripes in the lengthwise direction with respective colors arranged alternately in the crosswise direction, the pitch of the reinforcing wires in the lengthwise direction is identical with the pitch of luminescent pixels at the smallest or integer times the pitch, and the pitch in the crosswise direction is integer times the pitch of luminescent pixels.
Further, as a multiple luminescent area deposition mask, it is also known that if the mask member is stuck to a frame with n openings, productivity can be enhanced (for example, see Patent Document 3), but this technique is not effective for inhibiting the local bending of the mask member.
Known is another multiple luminescent area deposition mask in which a striped first mask member and a second mask member for defining the deposition range are overlaid on each other (for example, see Patent Document 4), but the technique cannot solve the problem addressed by this invention of to achieve no impact of the local bending of the mask member on the luminescent area. Moreover, since it is necessary to align the two mask members, i.e., the striped first mask member and the second mask member to the object for deposition, the technique is disadvantageous in view of productivity. Furthermore, the second mask member may cause shadowing and raise the risk of producing defective products.    [Patent Document 1] JP2000-160323A    [Patent Document 2] JP2000-12238A    [Patent Document 3] JP2003-152114A    [Patent Document 4] JP2003-68454A